This invention relates to the heating and cooling of solid materials by heat exchange with liquids as well as to the thermal decomposition, also called retorting, of solids such as oil shale, coal, industrial and municipal wastes, and the like, hereinafter referred to as carbon containing solids. Retorting involves the heating of such solids to temperatures at which they thermally decompose, releasing hydrocarbons, such as oil vapors and gases, which are then converted into fuels. Oil shale, which contains a minor amount of organic matter called kerogen and a major amount of mineral matter, is considered one of the best candidates of all carbon containing solids for the production of motor and heating fuels.
Processes for recovering hydrocarbonaceous products from raw oil shale are generally classified into four categories according to the method in which the heat is supplied. These categories are as follows: (1) heat is transferred from an external source through the walls of the retorting vessel; (2) heat is supplied by direct combustion in the retorting vessel; (3) heat is supplied by passing an external heated gas into the retorting vessel; (4) heat is supplied by introducing externally heated solids into the retorting vessel. It is noteworthy that the processes falling into category (1), to which the process of the invention belongs, have heretofore attracted very little interest. Nevertheless, category (1) has a number of advantages over other categories provided that its present drawbacks could be eliminated. This is achieved in this invention by a novel means for liquids to solids heat exchange whereby the solids are conveyed by an oscillating motion on a continuous conveying path while exchanging heat with suitable heat transfer media such as liquid metals. The conveying path preferably includes a retorting zone and a cooling zone in which heat is recovered from cooling of the oil shale and utilized for its retorting for substantial fuel savings.
The heat exchanger of this invention is applicable not only to retorting, but also to heating or cooling of solid materials in general. The liquid to solids heat exchange, and in particular, the exposure of a wall in an oscillating pan for a pressurized contact with a liquid through immersion in a liquid pool are distinctly different from the prior art.
Characteristically, the prior art comprises an oscillating chamber for this purpose. The liquid flows in the chamber, which may be rectilinear or helical, not unlike in a conduit while exchanging heat with the solid materials. U.S. Pat. No. 2,805,841 describes a typically illustration of such a chamber. In a heating application, referred to in the patent, albeit not shown on its drawings, the liquid flows inside the chamber by gravity while being pumped to or from an external heat source in order to transfer heat therefrom to the solid materials. The liquid of necessity flows by gravity because the pipes supplying the liquid cannot be connected to the chamber except by flexible piping connections. However, the latter are feasible only for liquid temperatures which are substantially below the peak retorting temperature. A submersed piping asembly for heating, as in this invention, is also precluded for the same reason, because this would necessitate flexible piping connections between the piping assembly inside the oscillating chamber and the piping supplying the heating fluid.
The drawbacks of the prior art are avoided in this invention. This is achieved by the immersion of an oscillating pan in a liquid pool which is contained in a reservoir. An advantage of the reservoir over the prior art is that liquid metals can be safely contained and heated therein by a fluid flowing inside a submersed piping assembly. The oil shale or other solid materials can therefore be heated to higher temperatures than heretofore possible. Another advantage over the prior art is that the immersion causes the liquid to exert an upward pressure on the pan which besides exposing the liquid for a pressurized contact with the pan is also a means to cancel out the "dead loads" due to the weight of the pan and the solid materials thereon.
Another drawback of the prior art which is eliminated by this invention results from the difference in temperature between the upper wall and the bottom wall of the chamber. The upper wall is exposed to solid materials and to a liquid whereas the bottom wall is exposed only to a liquid. The resulting temperature difference produces a difference in the thermal expansion of the walls, which induces stresses. Although the severity of the stresses will depend on a number of factors besides temperature, there is nonetheless a temperature limit beyond which the chamber will rupture. This drawback is avoided in this invention because the duct underneath the wall has no bottom. The oscillating pans have yet another advantage because they can form a continuous conveying path by overlapping one pan on top of another. Unlike in the prior art, the thermal expansion increases the amount of overlap but not the total length of the conveying path. The terminals of the conveying path stay therefore near their original location. A much longer conveying path and larger heating capacities as well as higher temperatures, than in the prior art, can thus be achieved.